The invention relates generally to spectrometers and in particular, a grating-based, integrated photonic, staring hyperspectral imager (HSI).
Spectral imaging involves collecting information about an image from across the electromagnetic spectrum. In its simplest form, a multi spectral imager (MSI) collects image information in several spectral bands within a specific wavelength range, for example, a visible camera acquires images at red, green, and blue wavelengths. Improvements in many technical fields have made possible the collection of a wider range of wavelengths. Multispectral systems can now collect light from not only the visible spectrum, but also, for example, the infrared spectrum. In addition, the spectrum to be collected can be divided into many more spectral bands (typically ten or fewer for multispectral systems). Such systems have evolved from several spectral bands (multispectral) to hundreds of bands (hyperspectral) and will continue to evolve into even finer spectral “binning” (ultraspectral). These sensors provide high resolution spectral coverage of wavelength ranges spanning the visible and near infrared (VNIR), short-wave infrared (SWIR) and long-wave infrared (LWIR), and the latest developments in this area have led to very compact, modular high-performance systems.
Interest in HSI systems has rapidly increased. HSI systems have demonstrated the capability to remotely detect subtle spectral features in reflected and emitted energy from the earth's surface and atmosphere. These features allow detection, identification, and characterization of materials without the need to spatially resolve materials of interest. This capability has proven crucial to a number of remote sensing applications, from natural resource monitoring to detection of military targets.
Prior art grating spectrometers include pushbroom and whiskbroom scanners. In a pushbroom scanner, the image of the target area is collected one row of image pixels at a time. The light from the pixels is dispersed by a grating and projected onto a 2-dimensional focal plane array (FPA) of detectors. One dimension corresponds to the spatial position of the pixel and the other dimension corresponds to the spectral content of the pixel. One drawback of a pushbroom scanner is that it needs the platform's motion to scan the target area and therefore has a time dependence for obtaining the area data. The diffraction gratings, FPAs and associated optics result in HSIs that are large and heavy. They are vulnerable to scattered light effects. In addition, it is difficult to implement enhanced techniques such as Time Delay Integration (TDI) since that would require multiple copies of the entire spectrometer.
A Fourier Transform Spectrometer (FTS) is type of spectrometer that does not use dispersive elements. An FTS gathers full spectrum data at varying optical path lengths of the internal spectrometer optics, a process which requires scan mechanisms internal to the device. It then uses a Fourier transform to turn acquired data into an actual spectrum.
Thus, there is a need for HSIs with reduced mass and volume, improved minimization of scattered light and TDI operation that are capable of being rapidly and inexpensively produced in large quantities.